Electrochemical process for producing chloric acid-alkali metal chlorate mixtures

ABSTRACT

A process for producing chlorine dioxide from an aqueous solution of chloric acid and alkali metal chlorate which is prouced in a electrolytic cell having an anode compartment, a cathode compartment and at least one ion exchange compartment between the anode and cathode compartments. The process includes the steps of feeding an aqueous solution of an alkali metal chlorate to the ion exchange compartment, electrolyzing an anolyte in the anode compartment to generate hydrogen ions, passing the hydrogen ions from the anode compartment through a cation exchange membrane into the ion exchange compartment to displace alkali metal ions and produce an aqueous solution of chloric acid and alkalimetal chlorate, passing the alkali metal ions from the ion exchange compartment into the cathode compartment, and finally passing the aqueous solution of chloric acid and alkali metal chlorate to a chlorate dioxide generator.

This application is a division of application Ser. No. 07/475,603, filedFeb. 9, 1990, now U.S. Pat. No. 5,084,148.

BACKGROUND OF THE INVENTION

This invention relates to a process for electrochemically producingchloric acid --alkali metal chlorate solutions. More particularly, thisinvention relates to the electrochemical production of chloricacid--alkali metal chlorate solutions suitable for the generation ofchlorine dioxide.

Chlorine dioxide has found wide use as a disinfectant in watertreatment/purification, as a bleaching agent in pulp and paperproduction, and a number of other uses due to its high oxidizing power.There is a variety of chlorine dioxide generator systems and processesavailable in the marketplace. Most of the very large scale generatorsemployed, for example, in pulp and paper production, utilize an alkalimetal chlorate salt, a reducing agent, and an acid in a chemicalprocesses employed also produce by-product salts such as sodiumchloride, sodium sulfate, or sodium bisulfate. In pulp and paper mills,the typical by-product is sodium sulfate (saltcake) which is convertedinto a sulfur salt of sodium in a high temperature boiler and used inthe paper process. Boilers require energy and the paper mills have alimited boiler capacity. Increasing the production of chlorine dioxidegenerally means increased capital investment to provide the added boilercapacity required to process the added amounts of saltcake by-productproduced. Thus a process which reduces the amount of a by-product salt,such as sodium chloride or sodium sulfate, produced while efficientlygenerating chlorine dioxide is commercially desireable.

U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumbergerteaches a process for producing chloric acid by passing an aqueoussolution containing from 0.2 gram mole to 11 gram moles per liter of analkali metal chlorate exchange resin at a temperature from 5° to 40° C.The process produces an aqueous solution containing from 0.2 gram moleto about 4.0 gram moles of HClO₃. This process requires the regenerationof the cationic exchange resin with acid to remove the alkali metal ionsand the treatment of disposal of the acidic salt solution.

K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989,describe a process for chloride dioxide which electrolyzes a chloricacid solution produced by passing an aqueous solution of an alkali metalchlorate through an ion exchange resin. The electrolyzed solutioncontains a mixture of chlorine dioxide and chloric acid which is fed toan extractor in which the chloride dioxide is stripped off. The ionexchange resin is regenerated with hydrochloric acid and an acidicsolution of an alkali metal chloride formed.

In U.S. Pat. No. 4,683,039, Twardowski et al describe a method forproducing chloride dioxide in which the chlorine dioxide is produced ina generator by the reaction of sodium chlorate and hydrochloric acid.After separating chlorine dioxide gas, the remaining sodium chloridesolution is fed to a three-compartment cell to form sodium hydroxide andan acidified liquor which is returned to the chlorine dioxide generator.

Each of the above processes produces a fixed amount and type ofby-product salt.

SUMMARY OF THE INVENTION

Now a process has been discovered which permits variability in thecomposition of a chlorate solution used in chloride dioxide generators.Further, the process permits a reduction in the amount of acid requiredand subsequently the amount of salt by-product produced in the chlorinedioxide generator. Still further, the process allows for the productionof an alkali metal hydroxide as a valuable by-product of acidicsolutions of alkali metal salts at reduced energy costs. In addition,the process results in the reduction of process steps and processequipment required for the production of chlorine dioxide.

These and other advantages are accomplished in a process forelectrolytically producing an aqueous solution of chloric acid andalkali metal chlorate in an electrolytic cell having an anodecompartment, a cathode compartment, and at least one ion exchangecompartment between the anode compartment and the cathode compartment,which comprises feeding an aqueous solution of an alkali metal chlorateto the ion exchange compartment, electrolyzing an anolyte in the anodecompartment to generate hydrogen ions, passing the hydrogen ions fromthe anode compartment through a cation exchange membrane into the ionexchange compartment to displace alkali metal ions and produce anaqueous solution of chloric acid and alkali metal chlorate, and passingalkali metal ions from the ion exchange compartment into the cathodecompartment.

More in detail, the novel process of the present invention and itsapplication in producing chlorine dioxide can be carried out inapparatus illustrated in the following FIGURES.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side elevational view of an electrolytic cellwhich can be employed in the novel process of the invention; and

FIG. 2 is a sectional side elevational view of an additionalelectrolytic cell which can be employed in the novel process of theinvention.

FIG. 3 is a diagrammatic illustration of a system which can be employedin the process of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shown an electrolytic cell 4 divided into anode compartment 10,ion exchange compartment 20, and cathode compartment 30 by cationpermeable ion exchange membranes 16 and 24. Anode compartment 10includes anode 12, and anode spacer 24. Anode spacer 14 positions porousanode 12 with respect to cation permeable ion exchange membrane 16 andaids in the disengagement of anolyte gas produced. Anoltye disengager 18comprises the disengagement of anolyte gas from the spent anolytesolution. Ion exchange compartment 20 includes spacer material 22 whichprovides a flow channel between cation permeable ion exchange membranes16 and 24 for the aqueous alkali metal chlorate solution. Cathodecompartment 30 includes cathode 32, and cathode spacer 34. Cathodespacer 34 positions cathode 32 with respect to cation permeable ionexchange membrane 24 and aids in the disengagement of catholyte gasproduced. The disengagement of catholyte gas from the spent catholytesolution is accomplished in cathode disengager 36.

In FIG. 2, electrolytic cell 4 has been expanded to include a second ionexchange compartment 40 which is positioned between anode compartment 10and ion exchange compartment 20. Cation permeable ion exchange membrane42 separates anode compartment 10 from ion exchange compartment 40. Thesodium chlorate feed solution enters the lower part of ion exchangecompartment 20 into the upper part of ion exchange compartment 40. TheHClO₃ /NaClO₃ product solution is recovered from the lower part of ionexchange compartment 40.

The flow direction in the ion exchange compartments can also bereversed, for example, with the solution from the top of ion exchangecompartment 40 being fed to the bottom of ion exchange compartment 20.The product solution then exits from the top of ion exchange compartment20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aqueous solution of an alkali metal chlorate is fed to the single ormultiple ion exchange compartments of the electrolytic cell. Suitablealkali metal chlorates include sodium chlorate, potassium chlorate andlithium chlorate. In order to simplify the disclosure, the process ofthe invention will be described using sodium chlorate, which is apreferred embodiment of the alkali metal chlorates. As shown in FIG. 3,the sodium chlorate feed solution may be prepared, for example, bydissolving crystalline sodium chlorate in water. Commercial sodiumchlorate is suitable as it has a low sodium chloride content and theformation of undesirable amounts of chlorine dioxide in the electrolyticcell is prevented. Aqueous sodium chlorate feed solutions which may beemployed contain any suitable concentrations of sodium chlorate, forexample, solutions having a concentration in the range of from about0.1% by weight to those saturated with NaClO₃ at temperatures in therange of from about 0° to about 100°, and preferably from about 15° toabout 80° C.

The novel process of the invention utilizes an electrochemical cell togenerate hydrogen ions that displace or replace a portion of the sodiumions present in the aqueous sodium chlorate solution feed stream.

The generation of hydrogen ions in the process of the present inventionin the anode compartment is accompanied, for example, by the oxidationof water on the anode into oxygen gas and H+ ions by the electrodereactions as follows:

    2H.sub.2 O→O.sub.2 +4H.sup.30 +4e.sup.-

The anode compartment contains an anolyte, which can be an aqueoussolution of any non-oxidizable acid electrolyte which is suitable forconducting hydrogen ions into the ion exchange compartment.Non-oxidizable acids which may be used include sulfuric acid, phosphoricacid and the like. Where a non-oxidizable acid solution is used as ananolyte, the concentration of the anolyte is preferably selected tomatch the osmotic concentration characteristic of the alkali metalchlorate solution fed to the ion exchange compartment to minimize waterexchange between the anode compartment and the ion exchange compartment.Additionally, an alkali metal chloride solution can be used as theanolyte, which results in a generation of chloride gas at the anode.Where a chlorine generating anolyte is employed, it is necessary toselect a cation exchange membrane as the separator between the anodecompartment from the ion exchange compartment which is stable tochlorine gas. The anode compartment may also employ as the anolyteelectrolyte a strong acid cation exchange resin in the hydrogen form andan aqueous solution such as deionized water.

Any suitable anode may be employed in the anode compartment, includingthose which are available commercially as dimensionally stable anodes.Preferably, an anode is selected which will generate oxygen gas. Theseanodes includes porous or high surface area anodes. As materials ofconstruction for the anodes, metals including platinum, gold, palladium,or mixtures or alloys thereof, or thin coatings of such materials onvarious substrates such as valve metals, i.e. titanium, can be used.Additionally oxides of iridium, rhodium or ruthenium, and the alloyswith other platinum group or precious metals metals could also beemployed. Commercially available oxygen evolution anodes of this typeinclude those manufactured by Englehard (PMCA 1500) of Eltech(TIR-(1200). Other suitable anode materials include graphite, graphitefelt, a multiple layered graphite cloth, a graphite cloth weave, carbon,etc.

The hydrogen ions generated in the anode compartment pass through thecation exchange membrane into the sodium chlorate solution in the ionexchange compartment. As a hydrogen ion enters the solution, a sodiumion is displaced and by electrical ion mass action passes through thecation membrane adjacent to the cathode compartment to maintainelectrical neutrality.

The novel process of the invention as operated results in the conversionof sodium chlorate to chloric acid over a wide range, for example, fromabout 1 to about 99.9%, preferably from about 5 to about 95, and morepreferably from about 15 to about 90%.

The sodium chlorate feed solution concentration, the residence time inthe ion exchange compartment as well as the cell amperage are factorsthat affect the extent of the conversion of sodium chlorate to chloricacid. Using very dilute solutions of sodium chlorate, high percentagesof conversion of NaClO₃ to chloric acid can be achieved, i.e. up to99.9% conversion. For a single pass flow through system, typicalresidence times in the ion exchange compartment are between about 0.1 toabout 120 minutes, with a more preferred range of about 0.5 to about 60minutes.

Thus the concentration of sodium chlorate in the solution fed to the ionexchange compartment and the flow rate of the solution through the ionexchange compartment are not critical and broad ranges can be selectedfor each of these parameters.

The novel process of the present invention is operated at a currentdensity of from about 0.01 KA/m² to about 10 KA/m², with a morepreferred range of about 0.05 KA/m² to about 3 KA/m².

Current efficiencies during operation of the process of the inventioncan be increased by employing additional ion exchange compartments, asillustrated by FIG. 2, which are adjacent and operated in a series flowpattern.

Adjusting the width of the ion exchange compartment can also alter theoperating cell voltage and current efficiency. The width, or spacebetween the cation exchange membranes forming the walls of the ionexchange compartment, is in the range of from about 0.1 to about 10, andpreferably from about 0.3 to about 5 centimeters.

In an alternate embodiment the ion exchange compartment contains acation exchange medium. Cation exchange mediums which can be used in theion exchange compartment include cation exchange resins. Suitable cationexchange resins include those having substrates and backbones ofpolystyrene based with divinyl benzene, cellulose based, fluorocarbonbased, synthetic polymeric types and the like. Where more than one ionexchange compartment is employed, inclusion of the cation exchangemedium is optional for each compartment.

Functional cationic groups on these mediums which may be employedinclude carboxylic acid, sulfonic or sulfuric acids, and acids ofphosphorus such as phosphonous, phosphonic or phosphoric. The cationexchange resins are suitably ionically conductive so that a practicalamount of current can be passed between the cation exchange membranesused as separators. Various percentage mixture of resins in the hydrogenform and the sodium form may be used in various sections of the ionexchange compartments on assembly to compensate for the swelling andcontraction of resins during cell operation. For example, percentageratios of hydrogen form to sodium form may include those from 50 to100%.

The use of cation exchange resins in the ion exchange compartment canserve as an active mediator which can exchange or absorb sodium ions andrelease hydrogen ions. The hydrogen ions generated at the anode thusregenerate the resin to the hydrogen form, releasing sodium ions to passinto the cathode compartment. Their employment is particularlybeneficial when feed dilute sodium chlorate solutions as they helpreduce the cell voltage and increase conversion efficiency.

Preferred as cation exchange mediums are strong acid type cationexchange resins in the hydrogen form as exemplified by low cross-linkedresins such as AMBERLITE® IRC-118 (Rohm and Haas Co.) as well as highercross-linked resins i.e. AMBERLITE® IRC-120. High surface areamacro-reticular or microporous type ion exchange resins havingsufficient ionic conductivity in the ion exchange compartments are alsoavailable.

Physical forms of the cation exchange resin which can be used are thosewhich can be packed into compartments and include beads, rods, fibers ora cast form with internal flow channels. Bead forms of the resin arepreferred.

Cation exchange membranes selected as separators between compartmentsare those which are inert membranes, and are substantially impervious tothe hydrodynamic flow of the alkali metal chlorate solution or theelectrolytes and the passage of any gas products produced in the anodeor cathode compartments.

Cation exchange membranes are well-known to contain fixed anionic groupsthat permit intrusion and exchange of cations, and exclude anions froman external source. Generally the resinous membrane or diaphragm has asa matrix, a cross-linked polymer, to which are attached charged radicalssuch as --SO₃ ⁻⁻ and/or mixtures thereof with --COOH⁻⁻. The resins whichcan be used to produce the membranes include, for example,fluorocarbons, vinyl compounds, polyolefins, hydrocarbons, andcopolymers thereof. Preferred are cation exchange membranes such asthose comprised of fluorocarbon polymers or vinyl compounds such asdivinyl benzene having a plurality of pendant sulfonic acid groups orcarboxylic acid groups or mixtures of sulfonic acid groups andcarboxylic acid groups. The terms "sulfonic acid group" and "carboxylicacid groups" are meant to include salts of sulfonic acid or salts ofcarboxylic acid groups by processes such as hydrolysis.

Suitable cation exchange membranes are readily available, being soldcommercially, for example, by Ionics, Inc., Sybron, by E. I. DuPont deNemours & Co., Inc., under the trademark "NAFION®", by the AsahiChemical Company under the trademark "ACIPLEX®", and by Tokuyama SodaCo., under the trademark "NEOSEPTA®". Among these are perfluorinatedsulfonic acid type membranes which are resistant to oxidation and hightemperatures such as DuPont NAFION® types 117, 417, 423, etc., membranesfrom the assignee of U.S. Pat. No. 4,470,888, and otherpolytetrafluorethylene base membranes with sulfonic acid groupings suchas those sold under the RAIPORE tradename by RAI Research Corporation.

The catholyte can be any suitable aqueous solution, including alkalimetal chlorides, and any appropriate acids such as hydrochloric,sulfuric, phosphoric, nitric, acetic or others.

In a preferred embodiment, deionized or softened water or sodiumhydroxide solution is used as the catholyte in the cathode compartmentto produce an alkali metal hydroxide. The water selection is dependenton the desired purity of the alkali metal hydroxide by-product. Thecathode compartment may also contain a strong acid cation exchange resinin a cation form such as sodium as the electrolyte.

Any suitable cathode which generates hydrogen gas may be used, includingthose, for example, based on nickel or its alloys, includingnickel-chrome based alloys; steel, including stainless steel types 304,316, 310, etc.; graphite, graphite felt, a multiple layered graphitecloth, a graphite cloth weave, carbon; and titanium or other valvemetals as well as valve metals having coatings which can reduce thehydrogen overvoltage of the cathode. The cathode is preferablyperforated to allow for suitable release of the hydrogen as bubblesproduced at the cathode particularly where the cathode is placed againstthe membrane.

Optionally a porous spacer material such as a chemically resistantnon-conductive plastic mesh or a conductive material like graphite feltcan be positioned behind the anode and/or the cathode to support theelectrodes and to permit the adjustment of the gap between the electrodeand the cation permeable ion exchange membrane, for example, when usinghigh open area expanded metal electrodes. The porous spacer materialpreferably has large holes for ease of disengagement of the gases fromthe anolyte and/or catholyte. A thin protective spacer can also beplaced between the anode and/or the cathode and the cation permeable ionexchange membranes. This spacer can be a non-conductive plastic or aporous conductive material like graphite felt. The cell may be operatedwith the electrode in contact with the thin protective spacer and theporous spacer material, or with the membrane in direct contact with theelectrode and with or without the porous spacer material.

It will be recognized that other configurations of the electrolytic cellcan be employed in the novel process of the present invention, includingbipolar cells utilizing a solid plate type anode/cathode or bipolarmembranes. For example, a bipolar electrode could include a valve metalsuch as titanium or niobium sheet clad to stainless steel. The valvemetal side could be coated with an oxygen evaluation catalyst and wouldserve as the anode. An alternative anode/cathode combination which iscommercially available is a platinum clad layer on stainless steel orniobium or titanium and is prepared by heat/pressure bonding.

The novel product solution contains chloric acid and alkali metalchlorate in a wide range of concentrations and ratios of chloric acid toalkali metal chlorate. For example, the solutions produced can providemolar ratios of chloric acid to alkali metal chlorate of from about0.1:1 to about 250:1. Where the product solutions are to be used in thegeneration of chlorine dioxide, suitable molar ratios of chloric acid toalkali metal chlorate of from about 0.3:1 to about 200:1, and preferablyfrom about 1:1 to about 100:1. These solutions are highly acidic andpermit a reduction in the amount of acid required in the generation ofchlorate dioxide in commercial processes which react a chlorate solutionwith an acid in the presence of a reducing agent.

Further, the chloric acid --alkali metal chlorate solutions produced aresubstantially free of chloride, sulfate, phosphate, or other anionicgroups which are present when an alkali metal chlorate is acidified withmineral or other acids used in the generation of chlorine dioxide.

Where desired, the chloric acid concentrations of these novel solutionsmay be increased, for example, by evaporation at sub-atmosphericpressures and temperatures of about 100° C., or less. For example, inthe range of from about 30° to about 90° C. Solutions containing up toabout 40% by weight of chloric acid may be produced in this manner.

As illustrated in FIG. 3, the product solution can be fed directly fromthe electrolytic cell to a commercial chlorine dioxide generator.Typical commercial processes are those which use sulfuric acid orhydrochloric acid with a reducing agent such as sulfur dioxide ormethanol in the presence of a salt such as sodium chloride. Commercialchloride dioxide processes which may use the aqueous solutions ofchloric acid and alkali metal chlorate of the invention include theMathieson, Solvay, R2, R3, R8, Kesting, SVP, and SVP/methanol, amongothers.

The novel process of the present invention permits the production ofsolutions having a wide range of concentrations of chloric acid andsodium chlorate for use in chlorine dioxide generators. The processpermits flexibility in the by-product salts produced as well as allowingthe recovery of energy costs by producing, for example, an alkali metalhydroxide solution by-product. Further the process reduces operatingcosts by eliminating process steps and equipment from processespresently available. In addition novel solutions are procuced having awide range of chloric acid and alkali metal chlorate concentrationswhich are substantially free of anionic or cationic impurities.

To further illustrate the invention the following examples are providedwithout any intention of being limited thereby. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE 1

An electrochemical cell of the type shown in FIG. 1 consisting of threecompartments machined from ultra high density polyethylene (UHDPE)including an anode compartment, a central ion exchange compartment, anda cathode compartment. The 1/2 inch (1.27 cm.) thick anode compartmentcontained a titanium mesh anode having an oxygen-evolving anode coating(PMCA 1500® Englehard Corporation, Edison, NJ). The anode was supportedand spaced apart from the UHDPE back wall using multiple layers ofpolyethylene mesh having 1/4 inch square hole and being 1/16 inch inthickness. A polyethylene mesh spacer was positioned between the anodeand adjoining membrane to provide an anode-membrane gap of 0.0625 inch(0.1588 centimeters). The anode compartment was filled with a 2.0percent by weight sulfuric acid solution. The 1/4 inch (1.27 cm.) thickcathode compartment contained a 304 stainless steel perforated platecathode mounted flush to the surface of the cathode compartment with thepolyethylene mesh spacers. The cathode was positioned in contact withthe adjacent membrane providing a zero distance gap. The cathodecompartment was initially filled with a sodium hydroxide solution (2l%by weight) as the catholyte. Separating the anode compartment from theion exchange compartment, and the ion exchange compartment from thecathode compartment were a pair of perfluorosulfonic acid cationpermeable membranes with a 985 equivalent weight, obtained from theassignee of U.S. Pat. No. 4,470,888. The ion exchange compartment was amachined 1/4 inch (0.625 cm) thick frame with inlet and outlet andcontained the polyethylene mesh spacers to distribute the chloratesolution as well as to support and separate the two membranes.

An aqueous sodium chlorate solution containing 20 weight percent ofNaClO₃ was prepared by dissolving reagent grade sodium chlorate indeionized water. During operation of the electrolytic cell, the chloratesolution was metered into the bottom of the ion exchange compartment ina signal pass process at feed rates varying from 7.0 g/min. to 14.4g/min. Electrolyte circulation in the anode and cathode compartments wasby gas lift effect only. The cell was operated employing a cell currentof 24.5 amperes at a current density of 1.20 KA/m². The cell voltagevaried according to the cell operating temperature. A sample of theproduct solution was taken at each flow rate, the temperature measured,and the product solution analyzed for chloric acid and sodium chloratecontent. The product solutions were colorless, indicating no chlorinedioxide was formed in the ion exchange compartment. The concentration ofthe sodium hydroxide catholyte during cell operation increased to 12percent by weight. The results are given by Table I below.

                                      TABLE I                                     __________________________________________________________________________            NaClO.sub.3 Feed                                                                           HClO.sub.3 --NaClO.sub.3 Product                         Cell                                                                              Cell                                                                              Flowrate                                                                             Product                                                                             HClO3                                                                              NaClO3                                                                             HClO3:NaClO3                                                                          Conversion                                                                           C.E.                                                                              Residence                                                                           KWH/TON               Volts                                                                             Amps                                                                              (gm/min)                                                                             Temp (C.)                                                                           Wt % Wt % Molar Ratio                                                                           % to HClO3                                                                           %   Time (min)                                                                          of                    __________________________________________________________________________                                                            HClO3                 5.00                                                                              24.5                                                                              14.40  30.0  5.96 12.49                                                                              0.60    38.00  69.50                                                                             11.38 2082                  4.87                                                                              24.5                                                                              12.35  42.0  6.51 11.80                                                                              0.70    41.00  65.20                                                                             13.27 2152                  4.76                                                                              24.5                                                                              10.00  45.0  7.24 10.88                                                                              0.84    45.60  58.60                                                                             16.39 2336                  4.50                                                                              24.5                                                                              7.17   50.0  8.34 9.49 1.11    52.60  48.50                                                                             22.86 2674                  4.44                                                                              24.5                                                                              7.00   54.0  8.43 9.38 1.13    53.10  47.80                                                                             23.41 2673                  __________________________________________________________________________

EXAMPLE 2

The electrochemical cell of FIG. 2 was employed having a second ionexchange compartment adjacent to the first ion exchange compartment. Theanode compartment containing the same type of anode used in Example 1was filled with a strong acid hydrogen form cation exchange resin(AMBERLITE® IRC-120 plus, Rohm & Haas Company) as the electrolyte. Aperfluorinated sulfonic acid-based membrane (Dupont NAFION® 417)separated the anode compartment from the first ion exchange compartment.The two ion exchange compartments were fully filled with AMBERLITE®IRC-120 plus cation exchange resin in the hydrogen form and wereseparated by a Dupont NAFION® 417 membrane. The same membrane wasemployed to separate the second ion exchange compartment from thecathode compartment. The cathode compartment contained a perforated 304stainless steel cathode, and was filled with a sodium form AMBERLITE®IRC-120 plus cation exchange resin. Both the anode compartment and thecathode compartment were filled with deionized water. The sodiumchlorate solution fed to the ion exchange compartments was prepared fromreagent grade sodium chlorate dissolved in deionized water to from a 16weight percent solution as sodium chlorate. The sodium chlorate solutionat 20° C. was fed to the bottom of ion exchange compartment 40 adjacentto the cathode compartment at a flow rate of 6.5 grams per minute. Thechloric acid--sodium chlorate solution flow from the upper part of ionexchange compartment 40 was routed into the bottom of ion exchangecompartment 20 adjacent to the anode compartment and collected from thetop of ion exchange compartment 20. The total residence time of thesolution in the ion exchange compartments was about 42 minutes.

During the operation of the cell, the cell current was set at a constant23.0 amperes for an operation current density of 1.5 KA/m². The cellvoltage stabilized at 9.60 volts, and the product temperature was 65° C.Circulation in the anode and cathode compartments of the electrolyte wasby gas lift effect and the liquid level of the gas disengagers was setat 3 inches (7.62 cm) above the height of the cell.

The product solution from the cell contained 11.44 weight percent asHClO₃ which represented a 90% conversion of the sodium chlorate tochloric acid. The current efficiency was determined to be 61.6% and thepower consumption was 4490 KWH/Ton of HClO₃. The product solution waslight yellow in color, indicating of presence of some chloride dioxideor chlorine in the chloric acid-sodium chlorate solution product.

What is claimed is:
 1. A process for producing chlorine dioxide whichcomprises:a) feeding an aqueous solution of an alkali metal chlorate toa first ion exchange compartment of an electrolytic cell having an anodecompartment, a cathode compartment and at least one ion exchangecompartment between the anode compartment and the cathode compartment,b) electrolyzing an anolyte in the anode compartment to generatehydrogen ions, c) passing the hydrogen ions from the anode compartmentthrough a cation exchange membrane into the first ion exchangecompartment to displace alkali metal ions and produce an aqueoussolution of chloric acid and alkali metal chlorate, d) passing alkalimetal ions from the first ion exchange compartment into the cathodecompartment, and e) reacting the aqueous solution of chloric acid andalkali metal chlorate with a mineral acid and a reducing agent togenerate chlorine dioxide gas.
 2. The process of claim 1 in which themolar ratio of chloric acid to alkali metal chlorate in the aqueoussolution of chloric acid and alkali metal chlorate is in the range offrom about 0.1:1 to about 250:1.
 3. The process of claim 1 in which theaqueous solution of chloric acid and alkali metal chlorate from thefirst ion exchange compartment is fed to the lower part of the secondion exchange compartment.
 4. The process of claim 1 in which the aqueoussolution of chloric acid and alkali metal chlorate from the first ionexchange compartment is fed to the upper part of the second ion exchangecompartment.
 5. The process of claim 1 in which the alkali metalchlorate is sodium chlorate or potassium chlorate.
 6. The process ofclaim 1 in which between the steps d and e are the following stepscomprising:a) feeding, in series flow, the aqueous solution of chloricacid and alkali metal chlorate from the first ion exchange compartmentto a second ion exchange compartment, b) removing the aqueous solutionof chloric acid and alkali metal chlorate from the second ion exchangecompartment.
 7. The process of claim 1 in which the aqueous solutionconsists of chloric acid and alkali metal chlorate having a molar ratioof chloric acid to alkali metal chlorate of from about 0.3:1 to about200:1.
 8. The process of claim 1 in which the mineral acid is sulfuricacid or hydrochloric acid.
 9. The process of claim 1 in which thereducing agent is methanol or sulfur dioxide.
 10. The process of claim 1in which an alkali metal chloride is added to the reaction mixture ofstep e.
 11. The process of claim 1 in which the aqueous solutionconsists of chloric acid and an alkali metal chlorate beingsubstantially free of anionic and cationic impurities, the solutionhaving a molar ratio of chloric acid to alkali metal chlorate of fromabout 0.1:1 to about 250:1.